Molybdenum (Mo), a refractory metal from Group VI of the Periodic Table, can be strengthened by (a) solid-solution strengthening additions, (b) precipitation or dispersion strengthening by second-phase particles, (c) strain hardening and grain size refinement, and (d) retaining a worked structure at high temperatures. For service at elevated temperatures (&gt;0.5T.sub.m, where T.sub.m is the absolute melting temperature) only a combination of (b), (c), and (d) has the potential for success.
Strengthening by second-phase particles can be both direct and indirect as demonstrated for the SAP (sintered aluminum product) and TD (thoria dispersion strengthened) Nickel and TD NICHROME.RTM. alloys. Direct strengthening is caused by particles acting as barriers to dislocation motion during deformation. Indirect strengthening is caused when a dispersoid-containing metal is thermomechanically processed, so that the particles help to develop and stabilize a worked structure. An additional strength increment resulting from the fine grain size and substructure can be achieved at high temperature if the structure is stable. Carbides have been the favored second-phase particle for strengthening molybdenum, as is evident in the commercially available Mo-TZM and Mo-TZC molybdenum alloys. Mo-TZM is an arc cast and powder metallurgy product containing 0.5% by wt. Ti, 0.08% Zr, 0.02% C, and the balance Mo. Mo-TZC is an arc cast and powder metallurgy product containing 1.25% by wt. Ti, 0.15% Zr, 0.15% C, and the balance Mo. However, these alloys do not possess sufficient creep or tensile strength to survive operating temperatures at or above 2400.degree. F. (0.55 T.sub.m of molybdenum) for long-term service applications. These alloys, strengthened by a combination of solid-solution strengthening additions and carbide particles (precipitation strengthening) plus thermomechanical processing, lose their desired mechanical properties in relatively short time periods above these temperatures. The carbide particles are not stable at high temperature and will coarsen and/or dissolve into solution, thereby negating their effectiveness as a strengthener.
The development of a thermally stable, high-strength, molybdenum alloy is highly desirable, and would be useful to extend the lifetime or replace other material systems for a number of important uses. A few of the important uses are for: electrical posts for lamp filaments, creep-resistant boats for nuclear fuel sintering, high temperature components for metal matrix composites for critical high temperature aerospace and space nuclear power and propulsion system components, and medium caliber gun barrel liners for use with high impetus propellants.
An alternative to carbide strengthening of refractory metals such as molybdenum, is strengthening by an oxide dispersion. Tungsten, molybdenum, or their alloys, which are processed using powder-metallurgy techniques, are excellent candidates for oxide dispersion strengthening (ODS) because these metals have very low solubility for oxygen and only moderate affinity to form oxides.
The first oxide dispersion strengthened tungsten alloys containing a coarse, thermodynamically stable oxide (e.g. thoria, ThO.sub.2) particle addition, were developed to improve electron emission properties for use as welding rod in the gas tungsten arc (GTA) and other welding processes. However, ODS tungsten alloys containing a finer thoria particle were produced with exceptional creep strength and microstructural stability (i.e. recrystallization temperature, grain growth, etc.) up to 80% of the melting temperature of tungsten (0.8 T.sub.m =4839.degree. F.). In fact, the creep strength (i.e., stress to give 1% creep strain in 1,000 hours) of thoriated tungsten alloys was measured to be up to five times higher than commercially-pure tungsten. These alloys are used in several high-temperature applications such as lamp filaments which have improved resistance to shock loading during service, rocket nozzle inserts, and missile nose tips.
Cold-worked pure molybdenum will exhibit poor long-term creep properties above 2000.degree. F. because the cold-worked microstructure is not stable and will recrystallize during service. Some improvements to heat and creep resistance properties of Mo alloys have been made by adding oxides of metals to molybdenum. U.S. Pat. No. 4,950,327 (Eck et al.) discloses a mechanical preparation method, or dry blended process, of preparing an ODS molybdenum alloy, wherein powders of Mo metal and up to 10% by weight of an oxide, or a low temperature decomposable hydroxide or carbonate oxide, are mixed. The resulting powder batch mixture is then cold isostatically pressed (CIP), hydrogen sintered, and processed to wire or sheet. As shown in Table 1 of the Eck et al. patent, the steady-state creep rate of this ODS molybdenum alloy was improved over commercially pure molybdenum. However, no data are given concerning the maximum rupture life and rupture ductility which are more meaningful properties for evaluating the alloy. The ODS molybdenum alloys made by the wet-doping process of the invention disclosed herein, containing 2-4 volume % of the oxides of La, Ce, and Th show improved creep rate and superior rupture life and ductility.
Another method of making a Mo alloy which is creep-resistant up to high temperatures is disclosed in U.S. Pat. No. 4,622,068 (Rowe et al.). The method is a wet-doping process wherein Mo metal or Mo oxide is mixed with the salt of a metal. Under the conditions of the process, a Mo alloy is produced which contains 0.2 to 1% by weight of the metal oxide. Lanthanum salts. are not disclosed, nor are oxide weights above 1% disclosed. No mechanical property test data for the alloy produced are given in the Rowe et al. patent.
Therefore there is a need for a molybdenum alloy which will have sufficient and stable creep strength, over a long period of time, and sufficient tensile strength, to be stable at operating temperatures above 0.55 T.sub.m of Mo, i.e. above 2400.degree. F. (1315.degree. C.).
It is an object of the present invention to provide a thermally stable molybdenum alloy at temperatures above 2400.degree. F.
It is another object of the invention to provide a molybdenum alloy with high tensile strength.
It is a further object of the invention to provide a molybdenum alloy which has improved creep resistance especially above 2400.degree. F.
It is yet another object of the invention to provide a molybdenum alloy with improved ductility.
The present invention provides a wet-doping process wherein salts of the metal lanthanum, cerium, thorium or yttrium are added to molybdenum oxide to produce a fine-grained oxide-dispersion strengthened (ODS) molybdenum alloy containing about 2-4% by volume (.about.1-4 weight percent) of the oxides of lanthanum, cerium, thorium or yttrium.